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J.N. Bregman (University of Michigan)
We obtain clues to the nature and evolution of ULX progenitors from their location in galaxies as well as from theoretical models. For the nearest galaxies (d < 20 Mpc), the luminous non-nuclear ULX sources generally lie within star-forming regions, such as spiral arms. This indicates that they are not long-lived phenomena (<3x107 yr), there must be ~103 such objects in a galaxy (but in a dormant state), and that their progenitors are massive stars. However, current theoretical models for massive stars do not produce black holes in excess of about 10 Msolar, so if ULXs are shown to have masses of 102-104 Msolar, it would pose a challenge in the area of stellar evolution. We review the ways in which intermediate mass black hole systems could be produced and show that there are substantial difficulties in feeding such systems once they are formed.